Treatments & enhancements

Heating is the oldest and most widespread gem treatment. SSEF places commercial heating of corundum in a range of roughly 700 to 1,800 °C, dividing low-temperature work below about 1,000 °C from high-temperature work above about 1,200 °C. Past that upper figure the fine rutile needles called silk dissolve and expansion cracks open around inclusions; GIA’s 2019 Madagascar heating experiments recorded deeper, more saturated blue in stones taken that high. Heat also converts amethyst to citrine, and GIA states that most citrine sold is heated amethyst because natural citrine is rare. GIA describes aquamarine as routinely heated to remove greenish tints. LMHC Information Sheet #15 of 2025 calls tanzanite commonly heated and adds that the treatment is sometimes not determinable.

Laboratories judge heating from altered inclusions, glassy residues in healed fissures and infrared spectra. LMHC member laboratories report either no indications of heating, abbreviated NH, or indications of heating, and grade heating residues in corundum as minor (FAH 1), moderate (FAH 2) or significant (FAH 3). Low-temperature heating can leave almost nothing to see.

Because heated ruby and sapphire are the norm, a report stating no indications of heating supports a higher price. No laboratory publishes the size of that difference, which varies with species, quality, size and origin.

Diffusion drives color-causing elements into a gem from an outside source at high temperature. The first commercial version, patented in 1975 by Carr and Nisevich at the Linde division of Union Carbide, diffused titanium into pale sapphire. GIA measured the resulting color layer at 0.15 to 0.42 mm in the stones it examined in 1990, so recutting or heavy repolishing can remove it, and immersion in methylene iodide shows color concentrated along facet junctions and around the girdle.

Beryllium diffusion became public early in 2002, when corundum treated in Thailand reached the market unannounced, first as orange to pinkish orange sapphires. GIA’s 2003 study found 10 to 35 parts per million atomic of beryllium in the diffused zone, with 10 to 15 ppma enough for strong yellow, and reported furnace runs at 1,780 to 1,800 °C lasting 25 to 100 hours that carried beryllium about 2 mm in 40 hours, deep enough to pass through a finished stone. Confirming it takes mass spectrometry such as LA-ICP-MS or SIMS.

LMHC Information Sheet #2 groups both methods under lattice diffusion, meaning diffusion of foreign elements into the crystal structure, and has member laboratories report that the color was modified by diffusion of a chemical element from an external source.

Heavily fractured, low-grade ruby can be made to look transparent by filling its fractures and cavities with lead glass. GIA’s 2006 study traced the material to very low grade translucent to opaque corundum from Andilamena, Madagascar. Treaters preform the rough, heat it at 900 to 1,400 °C, then mix it with powders based on lead and silica and reheat at about 900 °C. The glass reads about 1.75 to 1.76 on a refractometer, close enough to corundum that the filled fractures nearly disappear.

Under magnification the filler shows flattened and rounded gas bubbles and a flash effect, usually blue and sometimes orange, as the stone is turned. GIA’s durability tests found the glass intact to 600 °C but sweating out of fractures at 700 °C, etched by jeweler’s pickling solution, oven cleaner, ammonia, bleach and lemon juice, and unharmed by three hours of ultrasonic cleaning or repeated steam. Both GIA and the LMHC advise unmounting such stones before jewelry repair.

Naming follows the amount of glass. LMHC laboratories grade the filling F1, F2 or F3 and, for heavily filled material, give the species as manufactured product or corundum with glass. The FTC Jewelry Guides bar describing gem material mixed with any amount of filler by an unqualified stone name.

Almost every emerald has fissures reaching the surface, and filling them with a colorless substance of similar refractive index makes them far less visible. Cedarwood oil and Canada balsam are the traditional fillers; GIA’s 1991 survey of Brazilian practice found the epoxy resin sold as Opticon in routine use after cutting, and its 1999 study sorted 39 filling substances into oils, waxes, prepolymers and polymers. GIA’s buying guide states that most emeralds are treated with oil, resin or polymer.

The fillers are not permanent. GIA’s 2007 durability study found changes in about 35 percent of filled emeralds after mild exposure to ultraviolet light, display heat, chill-thaw cycles and drying, and concluded that none of the fillers tested survived every test. Ultrasonic cleaning, ethanol and acetone did most damage to liquid-filled stones.

The grading scale is published by the Laboratory Manual Harmonisation Committee. Information Sheet #5, version 6 of May 2025, sets four report states: no indications of clarity enhancement, and minor (F1), moderate (F2) or significant (F3) filling, judged on face-up appearance. GIA has classified emerald clarity treatment on that scale since early 2000. A stone the trade calls no oil is one whose report reads no indications of clarity enhancement. It fetches more because its look does not depend on a filler that can dry, leak or be stripped, and because the FTC and CIBJO both require the degree of treatment to be disclosed.

Two clarity treatments are specific to diamond. Laser drilling bores a channel from the surface to a dark inclusion, which is then dissolved or bleached with acid. GIA has recorded the practice since 1970 and describes it as an accepted trade practice from the early 1970s. The channel stays visible under magnification, and GIA treats drill holes as clarity characteristics and plots them on its reports. A later variant appeared in Israel in early 2000, called KM after kiduah meyuhad, Hebrew for special drill; it works through existing or induced fractures, so no surface drill hole shows and the path resembles a natural feather, the trade term for a fracture.

Fracture filling was developed in the 1980s by Zvi Yehuda of Ramat Gan, Israel, and announced by GIA in August 1987. The filler is a lead-bearing glassy material containing chlorine and oxygen, sometimes with bismuth. Filled breaks show a flash effect, yellowish orange in darkfield turning vivid blue as the stone is rotated, along with flow structure and flattened bubbles. GIA warned in 1989 that such stones should never meet a jeweler’s torch.

GIA grades laser-drilled diamonds because drilling is permanent, but issues no clarity grade for fracture-filled diamonds, since the glass is unstable and can be damaged by steam, acid or ultrasonic solutions.

High-pressure, high-temperature (HPHT) treatment anneals a diamond under conditions near those in which it formed. General Electric and Lazare Kaplan International announced the commercial process on 1 March 1999, and early stones carried the girdle inscription GE POL. GIA describes HPHT as annealing in a press to alter color, often turning brownish diamonds yellow or green; on rare nitrogen-poor type IIa stones it removes brown, and the 2000 study of GE POL material recorded stones moving from the N–O range and Fancy Light brown to D–H. GE has never published the pressures and temperatures it uses. GIA grades HPHT-treated diamonds because the change is stable.

Irradiation creates color centers, lattice defects that absorb light. GIA says high-energy electrons are the most common source, and that annealing, controlled heating, often follows to shift the result. The US Nuclear Regulatory Commission calls topaz the most commonly treated stone, notes that blue topaz occurs in nature but is very rare, and states that most blue topaz on the market has been irradiated.

Neutron and accelerator bombardment can leave a stone slightly radioactive; gamma irradiation does not. NRC rules require distributors to hold a license and to run radiological surveys before sale, and stones are set aside, typically for a couple of months, while the activity decays.

Some treatments work at the surface or through a material’s pores. GIA describes a diamond coating as an ultra-thin layer of foreign material applied to mask the original color or add one, warns that coatings scratch, wear off and are damaged by heat and chemicals, and issues no report for a coated diamond. Because the layer is not permanent, the FTC Jewelry Guides require it to be disclosed.

Dyeing needs somewhere for color to go. Gem-A notes that stones are soaked in pigment solutions, sometimes after heating or chemical attack has opened fractures and pore spaces, and lists jadeite, opal, lapis lazuli, pearls, corundum and quartz among commonly dyed materials, along with quartzite, magnesite and marble used as simulants. Idar-Oberstein has colored agate by inorganic chemistry for generations, and a sugar route deposits carbon to make black agate. Dye concentrates in cracks and pores, and an acetone swab can pick up the color.

Bleaching removes color. GIA states that akoya and freshwater cultured pearls are routinely processed, bleaching being the most common step. In jadeite, acid bleaching strips iron compounds from fractures and grain boundaries and leaves material so brittle that samples crumbled under finger pressure, so polymer impregnation follows. The LMHC reports the result as impregnated with resin or wax, known in the trade as B-jade, and dyed jadeite as C-jade, and says light surface waxing need not be declared.

Porous gems that would otherwise absorb skin oils or look chalky are often filled. Waxing and oiling are long-standing surface treatments that improve durability and appearance. Stabilization goes further: resin is forced into the pores under vacuum and pressure. GIA’s 2021 study of resin-filled turquoise measured surface porosity falling from about 1.9 percent to 0.7 percent, with color improving because less light scatters, and identified the resin by an infrared band near 1730 cm⁻¹ and by strong blue fluorescence under long-wave ultraviolet. A separate proprietary process, the Zachery treatment, cuts porosity and improves polish while leaving standard gemological properties much as they were; GIA reported in 1999 that millions of carats had been treated over the previous ten years and that treated stones carry significantly more potassium, detectable by EDXRF.

Opal is treated for other reasons. The LMHC defines hydrophane opal as opal that absorbs liquid to the point of changing its appearance and possibly its weight, and warns that the immersion test can itself crack a stone. GIA has documented oil and Opticon forced into hydrophane opal under vacuum at about 80 °C. In the sugar and acid treatment, applied to porous Andamooka matrix opal from Australia, a hot sugar bath followed by acid deposits carbon in the pores, darkening the background so the play-of-color stands out.

Two codes set out what sellers must say about treatments. The CIBJO Blue Books, published by the World Jewellery Confederation, cover diamonds, colored gemstones, pearls, coral, precious metals, gemological laboratories and responsible sourcing. CIBJO calls them living documents reviewed annually and presents them as filling the near absence of ISO standards for the trade; it also records that a Munich court relied on them in 2004 when it stopped a seller using the term cultured diamonds. The Gemstone Book of December 2022 defines a treatment as a practice that artificially changes a gem’s appearance or durability, requires disclosure before the sale is completed and on invoices and in advertising, and sets codes including O for oil-filled, RES for resin-filled and F for fracture or glass filled. The 2024 Pearl Book lists bleaching, dyeing, irradiation, filling, coating, oiling, waxing and luster treatment among treatments that must be disclosed.

In the United States the reference is the FTC’s Guides for the Jewelry, Precious Metals, and Pewter Industries, 16 CFR Part 23, last amended on 16 August 2018. Section 23.24 makes it deceptive to withhold a treatment that is not permanent, creates special care requirements or significantly affects value. Disclosure belongs at the point of sale, and for catalog or online selling in the solicitation itself.